A need has long existed for lasers that can be tuned to any wavelength within a given band. Until the present invention, it has only been possible to tune lasers, for example diode lasers operating in the 1.3 micron band or the 1.55 micron band, that are the focus of experimentation for optical fiber communication systems, over a range of one-thousandth of a micron, that is to say a range of perhaps a nanometer. Since such optical fiber communication systems typically employ a multiplicity of individual channels, each separated from its neighboring channels by at least a nanometer, a multichannel system has required a multiplicity of different lasers, each one fabricated to operate at one of the required wavelengths. Three types of lasers have been used in such applications. There are: the distributed feedback (DFB) laser, the Fabry-Perot (FP) laser, and the Distributed Bragg Reflector (DBR) laser. In the DFB laser, an internal periodic feedback structure establishes the wavelength of operation. In the Fabry-Perot (FP) laser, the two facets of the diode, the rear and the front or emitting surface, are cleaved to establish the dimensions of the structure such that a primary longitudinal mode of resonance will exist at the desired wavelength. In the DBR laser, a periodic feedback structure external to the laser diode is used to establish the operating wavelength. Hybrid types of lasers also have been proposed for such uses. The hybrid lasers use various combinations of these principles to establish the operating wavelength.
A problem with Fabry-Perot types of lasers is that their structure supports a multiplicity of resonant frequencies, so that these lasers output a band of individual wavelengths. Such lasers cannot therefore be used in wideband applications, since a single, narrow linewidth output is needed for wideband applications.
The other laser diode types previously mentioned can be fabricated to produce essentially single line (single-wavelength) output that is suitably narrow, but in order to change their output wavelength, the spacing of their periodic structures must be changed. Where an internal periodic structure is involved, the cost of tailoring that structure to a specific wavelength is quite high. Therefore, present manufacturing practice is to design the structure for a nominal wavelength, typically at the center of a desired band, and to take advantage of the manufacturing tolerances to produce a range of laser diodes with individual output wavelengths at various points centered about that band center. Some of those wavelengths are the ones desired, so those lasers can be marketed. However, most are not, so the effective yields are low and the prices correspondingly high.
This fact has stimulated interest in laser diodes that employ external wavelength control structures such as feedback gratings. However, such designs as have been suggested have had inherently narrow tunability, or, at best, discontinuous, with small regions of smooth tunability alternating with regions of instability.
The problem with those designs is that they fail to provide a means of adjusting cavity length in proportion to a change in the periodicity of the feedback structure. An illustration of such a design is contained in U.S. Pat. No. 4,786,132. The apparatus of that patent proposes to tune the output of a laser diode by incorporating into the effective laser cavity a feedback grating whose line spacing can be changed. The patent proposes a hybrid device, a Fabry-Perot diode laser in which only the rear surface is reflective to form one end of a lasing cavity. The other surface is coated to be antireflective, and has its output region connected to an optical fiber, comprising a central fiber and a surrounding cladding, the central fiber of which is coupled to a feedback grating. The effective center of reflection of that grating is the determinant of total cavity length, measured from the rear surface of the diode itself. U.S. Pat. No. 4,786,136 states the well-known fact that in order for the device to exhibit lasing, the total round trip path length, from the diode's rear surface, out to the grating's effective center of reflection and back to the diode's rear surface must be a whole number of wavelengths, or a whole number of wavelengths plus one-half wavelength, depending on the phase shift upon reflection.
However, the patent also states that the laser's output wavelength can be tuned by changing the effective periodicity of the feedback grating alone, and suggests two methods for accomplishing such a change. The patent does not admit the necessity of also changing the cavity length in proportion to the change in feedback wavelength, nor does it suggest any means of achieving this essential requirement.
In the absence of some such means, any wavelength tuning that way be achieved with the patent's proposed configurations will be discontinuous, with regions of instability comprising much of the intended tuning range.
It as an object of the present invention to provide means for simply and simultaneously adjusting both cavity length and the feedback wavelength to a laser, said adjustment being precisely proportioned to maintain stable laser operation over a broad tuning range.
Another need that has long existed in connection with optical fiber communication systems is a means for precisely fixing the wavelength of the transmitted, or upstream signal with reference to the received, or downstream, signal. Typical optical fiber communication systems use what is known as wavelength division multiplexing (WDM) in order to combine a multiplicity of channels for transmission on a single optical fiber between a central office and a geographically clustered group of subscribers remote from the central office. Typically, different wavelength carriers, one per subscriber, are modulated at the central office with the signals addressed to each subscriber. These several wavelengths are then multiplexed onto a single optical fiber by a wavelength selective device, for transmission to a remote distribution center. There the individual wavelengths are demultiplexed by a similar device that operates in reverse fashion to separate the individual wavelengths, steering them onto individual optical fibers that are each routed to an associated subscriber. These downstream wavelengths are typically centered around a wavelength of 1.3 microns, to take advantage of the low loss experienced in that wavelength band by signals propagating along typical optical fibers.
In such WDM systems, the upstream carrier that is modulated with signals originating at the subscriber's premises and sent to the central office for routing to other subscribers, is typically, using present technology, in the 1.55 micron band. That band is chosen in part because it is sufficiently distant from the 1.3 micron band to permit its separation from the latter band by wavelength-selective devices, where the signals in the two bands may share occupancy of certain network components. Another reason for choosing the 1.55 micron band is that signals propagating in optical fiber at that wavelength experience the least dispersion, or differential delay, between their high-frequency and low-frequency components. This is an important consideration for wideband signal transmission.
In a typical WDM system, the downstream wavelengths sent to a remote distribution center for distribution to the group of subscribers it serves, are hierarchically arrayed with uniform spacing. Thus if the channel spacing is five nanometers, and subscriber 1 receives a signal at a wavelength of 1.3 microns, then subscriber 2 will receive a signal at a wavelength of 1.305 microns, subscriber 3 will receive a signal at a wavelength of 1.310 microns, and so forth.
In such a typical system, the same hierarchical order must be maintained in the upstream direction, but separated by the 250 nanometer separation between the 1.3 micron and 1.55 micron bands. Thus, subscriber 1 should transmit upstream at a wavelength of 1.550 microns; subscriber 2 at a wavelength of 1.555 microns; subscriber 3 at a wavelength of 1.560 microns, and so forth. Channel spacings can be closer together, perhaps on the order of one nanometer, to permit more channels to be multiplexed onto a single optical fiber. Regardless of the channel spacing, the upstream channels must maintain the hierarchical relation of their respective downstream channels. With present laser sources, it is very costly to effect such hierarchical matching. As previously noted, laser manufacturers experience low yields at any specific wavelength other than the actual wavelength for which a laser is designed.
It is therefore a further object of this invention to provide a means of wavelength comparison in the form of a wavelength comparator. The input to the comparator will be two signals: a sample of a reference wavelength and a sample of the wavelength of the tunable laser that is the first object of this invention. The output of this comparator will be an error signal that can be used to tune the tunable laser. A sample of the output of that tunable laser will be fed back to the wavelength comparator, closing a servo loop so that the laser is gradually tuned to the point where its output wavelength lies at a fixed, desired offset from the reference wavelength. Since the loop remains closed, the system will automatically maintain the wavelength of the tunable laser at the desired offset. In the case of the WDM systems just discussed, the reference wavelength would be a sample of the received downstream signal and the desired offset wavelength would be 250 nanometers. A yet further object of the present invention is to devise a network architecture employing these novel devices.